1. Field of the Invention
The present invention relates to an air flow measuring device having a first sub-passage configured to introduce therein a part of air flowing in an interior of a duct, a second sub-passage configured to introduce therein a part of air flowing in the first sub-passage, and a flow amount sensor located in the second sub-passage to measure a flow amount of air.
2. Description of the Related Art
An air flow measuring device described in U.S. Pat. No. 7,089,788 (corresponding to JP 2005-140753A) is provided for measuring a flow amount of intake air flowing into an internal combustion engine. As shown in FIG. 3, the air flow measuring device includes a sensor body 110 disposed in an intake air passage 100 of the internal combustion engine. The sensor body 110 is provided with a first sub-passage 120 into which a part of air flowing in the intake air passage 100 is introduced, and a second sub-passage 130 into which a part of air flowing in the first sub-passage 120 is introduced. A flow amount sensor 140 is located in the second sub-passage 130. The second sub-passage 130 is formed into approximately a U-shape around a partition wall 150. Furthermore, a protrusion plate 150a is provided to protrude from the partition wall 150 into the first sub-passage 120 so that the dynamic pressure of air flowing in the first sub-passage 120 is received by the protrusion plate 150a. Therefore, a pressure difference between an inlet side and an outlet side of the first sub-passage 120 is increased, and thereby it is possible to introduce a sufficient flow amount of air into the second sub-passage 130 from the first sub-passage 120.
In the flow amount measuring device, when dust contained in air passes in the first sub-passage 120, the dust collides with the protrusion plate 150 protruding into the interior of the first sub-passage 120, bounces on the protrusion plate 150 and enters the second sub-passage 130, as in the arrows shown in FIG. 3. The dust entering the second sub-passage 130 may collides with the flow amount sensor 140 located in the second sub-passage 130. In particular, when a thin film-like measuring element is used in the flow amount sensor 140, the measuring element is easily damaged by the collision with the dust.